Methanol, the simplest alcohol, with a chemical formula of CH3OH, is a light, volatile, colorless, flammable liquid. A polar liquid at room temperature, methanol finds use as an antifreeze, solvent, fuel, and as a denaturant for ethanol. It is also used for producing biodiesel via a transesterification reaction.
The largest use of methanol, however, is in the manufacture of other chemicals. About forty percent of methanol is converted to formaldehyde, and from there into products as diverse as plastics, plywood, paints, explosives, and permanent-press textiles.
Methanol is also used on a limited basis as fuel for internal combustion engines. The use of methanol as a motor fuel received attention during the oil crises of the 1970's due to its availability, low cost, and environmental benefits. However, due to the rising cost of methanol and its corrosivity to rubber and many synthetic polymers used in the auto industry, by the late 1990's automakers had stopped building vehicles capable of operating on either methanol or gasoline (“flexible fuel vehicles”), switching their attention instead to ethanol-fueled vehicles. Even so, pure methanol is required as fuel by various auto, truck, and motorcycle racing organizations.
In 1923, German chemists Alwin Mittasch and Mathias Pier, working for BASF, developed a process for converting synthesis gas (a mixture of carbon monoxide, carbon dioxide, and hydrogen) into methanol. The process used a chromium and magnesium oxide catalyst and required extremely vigorous conditions—pressures ranging from 50 to 220 bar, and temperatures up to 450° C. A patent (U.S. Pat. No. 1,569,775) covering this process was issued on Jan. 12, 1926.
Modern methanol production has been made more efficient through the use of catalysts (typically copper) capable of operating at lower pressures. The modern low-pressure methanol (LPM) production process was developed by ICI in the late 1960s, with the technology now owned by Johnson Matthey (London), a leading licensor of methanol technology.
The production of synthesis gas (“syngas”) via steam reforming of natural gas is the first step in many processes for methanol production. At low to moderate pressures and at high temperatures around 850° C., methane reacts with steam on a nickel catalyst to produce syngas according to the following reactions:CH4+H2O→CO+3H2 CO+H2O→CO2+H2 This reaction, commonly referred to as “steam methane reforming” (SMR) is highly endothermic, and maintaining reaction temperature by external heating is a critical part of the process.
The syngas is then compressed and reacted on a second catalyst to produce methanol. Today, the most commonly used catalyst is a mixture of copper, zinc oxide, and alumina first used by ICI in 1966. At 50-100 bar and 250° C., it can catalyze the production of methanol from syngas with high selectivity:CO+2H2→CH3OHCO2+3H2→CH3OH+H2O
The production of syngas from methane produces 3 moles of hydrogen gas for every mole of carbon monoxide (and 4 moles of hydrogen per mole of carbon dioxide), while the methanol synthesis reaction consumes only 2 moles of hydrogen gas per mole of carbon monoxide (and 3 moles of hydrogen gas per mole of carbon dioxide). In both reaction pathways, one more mole of hydrogen is generated than is needed for methanol synthesis. This excess hydrogen occupies capacity in both the compressor train and the methanol reactor. As a result, the methanol production process is inefficient, resulting in unnecessary costs due to increased compressor power requirements and less than optimum methanol yields. Reactants are lost when excess hydrogen is purged from the synthesis loop and used as fuel for the reformer.
FIG. 1 is a schematic showing a conventional process for methanol production. Feed streams of natural gas 101 and steam 102 are fed into reformer 103, resulting in the production of syngas stream 104. Syngas stream 104 is then passed to compression chain 105 (typically comprising at least make-up compressor 105a and recycle compressor 105b) to produce high-pressure gas stream 106. High-pressure stream 106 is then passed to methanol synthesis reactor 107 to produce reaction product stream 108, containing methanol and unreacted syngas. This stream 108 is then routed to condenser 109, from which condensed stream 110, containing methanol and water, drops out. Overhead stream 111, containing unreacted syngas and enriched in hydrogen and inerts (methane and possibly nitrogen), is then split into purge stream 112 and recycle stream 113, which is routed back to the recycle compressor 105b, where it is combined with fresh feed.
It would be desirable to provide an improved methanol production process that is more efficient, with reduced compressor power requirements and/or improved methanol product yield.